Derivative Instruments and Hedging Activities - Amounts included in Accumulated Other Comprehensive Income (Details 4) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2012	Dec. 31, 2011
Amounts included in accumulated other comprehensive income for derivatives designated as hedging instruments
Unrealized gain (loss) on cash flow hedge included in other comprehensive income	$ (1,185)	$ (932)
Interest Rate Swap [Member] | Cash flow hedge [Member]
Amounts included in accumulated other comprehensive income for derivatives designated as hedging instruments
Unrealized gain (loss) on cash flow hedge included in other comprehensive income	$ (1,185)	$ (932)